1. Field of the Invention
This invention relates generally to power amplifiers, and more particularly to the DC-DC conversion of power amplifiers using an RF input.
2. Description of the Related Art
Power amplifiers are sized to deliver a certain maximum power at maximum efficiency for a nominal supply voltage (in the case of Li Ion based battery cells, this voltage is about 3.6V). The load presented to the amplifier is fixed as the load that delivers the best performance at the maximum power which means that the output swing of the amplifier makes use of all the available range of the supply voltage. As the power delivered decreases, the load remains the same which means that the voltage swing at the output of the power amplifier decreases (as P=V2/R). With a smaller swing, some of the available supply voltage range is wasted and consequently some power is wasted. If the supply is lowered using an efficient DC-DC converter, the output swing will now take up the available supply range and so there is no power wasted.
Also, in some power amplifier technologies (CMOS being and example) it is advantageous so have bias voltages for some of the transistors that are outside the range of the supply voltages. These voltages require a DC-DC converter to generate and this DC-DC needs to have a high efficiency.
Most power amplifiers used in cell phones have, until recently, been built using GaAs as the substrate material with HBT (heterojunction). Although switching DC-DC converters have been used with GaAs HBT power amplifiers, they have been on a separate silicon die and include their own clock. There is also no need to cascade the transistors of a GaAs HBT power amplifier, and so there has been no need to generate voltages higher than the power supply which means that GaAs HBT power amplifiers do not contain charge pumps.
A DC-DC converter switches a semiconductor switch device to convert a DC voltage, and conducts feedback control to maintain an output voltage supplied to a load at a certain value. The device requires a reactive element (inductor or capacitor) to store energy during one part of the switching, when the semiconductor device is on, and release it back when the semiconductor device is off. In the case where a lower voltage is desired, energy is stored in the reactive element for the same ratio of the full cycle as the ratio of the input voltage to output voltage, the current required by the load during the off portion being stored during the on portion of the cycle. In the case of a need to generate a higher voltage than the input voltage, one terminal of the energy storage reactance is switched between a lower voltage and a higher voltage to drive the voltage at the output higher. All forms of converter require a clock for switching. Some form of low pass filtering is also required to ensure that the output is relatively smooth with little ripple.
A step-down (higher voltage to lower voltage) is known as a “buck converter”. A step-up converter is known as a “boost converter”. There are also “buck-boost” converters that in one mode will operate as a buck converter (step a voltage down) and in another mode will operate as a boost converter (step a voltage up). Buck, boost and buck-boost converters use an inductor to store energy in the magnetic domain and can reach extremely high efficiencies in the region of 95% or more while delivering large amounts of current (a typical cellphone application would require currents measured in the Amperes during operation). The inductor needs to have low loss and be able to handle a large current. For efficiency reasons the DC-DC trends to be driven at relatively low frequency (several mega-Hertz) to limit the energy wasted in switching and so the inductor needs to have a high value to be able to store a lot of energy during the limited switcher on period. These factors lead to a need for a physically large inductor which in a cellphone application where space is at a premium.
A charge pump DC/DC converter is a power supply circuit that provides a regulated output voltage to a load from an input voltage source. One type of charge pump DC/DC converter is a switching DC/DC converter power supply that uses switches to convert the input voltage to a regulated output voltage. The switches are operated in sequence to first charge a capacitor from the input voltage and then transfer the charge to the output. An inverting charge pump operates by charging a “pump” capacitor during a clock's first half-cycle, or “pumping phase”, to the level of a source voltage. During the clock's second, non-overlapping half-cycle, or “transfer phase”, the pump capacitor is disconnected from the source and connected, with its polarity switched, to a second “reservoir” capacitor, thereby “pumping” charge to the reservoir capacitor and providing an output which is approximately the negative of the input voltage.
Similarly, with a minor rearrangement of the pump's switching elements, a step-up converter is produced. During the clock's first half-cycle the pump capacitor is charged to the level of the source voltage. During the clock's second half-cycle, the pump capacitor's positive side is disconnected from the source and its negative side, which had been connected to ground during the first half-cycle, is connected to the source. The positive side, now at twice the source voltage, is connected to the reservoir capacitor, thus charging it to twice the source voltage.
Other exemplary switch network configurations are directed to the generation of different output voltages as a function of the source voltage, e.g., a selectable bipolar doubled output voltage.
Although the charge pump requires few parts and no inductors, its main drawback is that the capacitors used are not able to store significant amounts of energy and so the load that the charge pump can drive is limited. However in applications where the load is purely capacitive and does not vary, the charge pump is a good solution for generating voltages. The gate of a MOSFET is such a purely capacitive load, and so a charge pump is a good solution for generating bias voltages for cascodes, wells, or even RF transistors.
In power amplifiers it is often necessary to generate voltages that are higher, or potentially lower, than the supplies in a manner that is efficient. For this reason, DC-DC converters, whether inductor based or charge pump based, are useful. This is particularly the case for power amplifiers made using a CMOS process as they tend to have a more complicated structure than those made in other processes and require voltages outside the range of the power supply voltage supplied to the part.